Receptacles used for weighing-in, such as laboratory containers, bottles, beakers, shallow dishes, flasks, test tubes and the like belong to the known state of the art. They serve for example as target containers in dosage-dispensing processes where prescribed quantities of substances, for example in the form of powders or pastes, are dispensed manually. Subsequently, the substances which are in powder- or paste form are dissolved in the weighing receptacle by adding a solvent and are for example analyzed in an HPLC (high-performance liquid chromatography) apparatus. HPLC instruments are based on a relative method of measurement and therefore need to be calibrated with a known standard solution before the actual analysis of an unknown substance is performed. In this process it is particularly important to know the exact concentration of the standard solution, as the measurement uncertainty of the result of the analysis cannot be better than the uncertainty of the stated composition or concentration of the standard solution that is being used. Solutions are normally prepared with a mix ratio of 1:1000 to 1:10,000, and the concentration is stated in milligram per liter. Typically, the substance is weighed in as accurately as possible in a calibrated weighing receptacle, i.e. a receptacle whose volume is defined by a calibration mark. Next, the receptacle is filled with solvent up to the calibration mark.
Given that substances in powder- or paste form are in most cases measured out manually, and since the minimum amount of substance that can be dispensed manually limited by natural factors such as, e.g., a steady hand of the person that does the dispensing, the shape of the spatula, the adhesive and cohesive properties of the substance particles and the like, a relatively large quantity of the solution, for example 100 ml, will have to be prepared in order to meet the precision required of the mix ratio. On the other hand, only an amount of, e.g., 2 ml of the prepared solution is used for the HPLC analysis, while the large leftover quantity will have to be safely disposed of.
A weighing receptacle used for preparing a solution accordingly has large dimensions, as the substance is preferably dispensed into the same receptacle in which the solution is prepared by adding the solvent.
In gravimetric measuring instrument with a high resolution of the weighing result, i.e. instruments used to weigh quantities in the milligram- or microgram range, the maximum load or maximum weight that can be set on the load receiver is not the only quantity that is subject to physical limitations. The size of the weighing receptacle likewise has a considerable influence on the weighing result. The problem stems in particular from the fact that even minimal differences in temperature will cause changes in air buoyancy of the weighing receptacle. For example, with a temperature rise of 0.1° C. the material of a 100 ml glass flask will expand to an extent that a 40 microgram deviation can be observed as a result of the increased air volume displaced by the flask. The manufacturing tolerances of weighing receptacles of this kind in regard to their weight (tare weight) and their behavior under temperature fluctuations make it impossible to find a practical way of compensating for the effects of air buoyancy. It is therefore hardly feasible to achieve a high level of precision in dispensing very small substance quantities into large weighing receptacles.
As the manufacturing costs and the costs for the safe disposal of waste materials are rising, the aim in the preparation of solutions is to reduce their quantities as much as possible. It would therefore suggest itself to use balances with a higher measurement resolution in order to be able to weigh even smaller substance quantities with a higher degree of precision, so as to achieve the same mix ratio with smaller amounts of substance. However, as explained above, the size of the weighing receptacle limits the feasibility of this approach. A possible starting point for solving the buoyancy problem is therefore the idea of reducing the weighing receptacle.
In a rudimentary form, this concept is being practiced in laboratories in that the substance is not dispensed directly into a large and heavy weighing container, but is measured out first into a lightweight weighing receptacle such as for example a weighing boat or a weighing paper. Next, the weighing paper is rolled into a funnel. This funnel or the weighing boat is then set into the opening of a container and the substance is flushed into the container with a solvent. However, this procedure has several drawbacks and risks.
The biggest drawbacks are in the achievable degree of precision and in the danger of contamination by substances that are hazardous to human health and to the environment. In the subsequent rolling of the paper and also when the paper or weighing boat holding the substance is transferred from the weighing pan to the container, it can happen that substance gets spilled. Furthermore, the work area and its occupant can be contaminated by health-endangering substances, if the latter include for example volatile microscopic particles. When the paper or weighing boat is rinsed with the solvent, particles can remain stuck to the surface, particularly in places that were not wetted by the liquid at all or only insufficiently, so that the solution will differ from the prescribed concentration. One should also not disregard the risk that the substance could become contaminated during the transfer by impurities in the ambient air.
It is therefore an object to provide a weighing receptacle for the weighing-in of substances, wherein the receptacle itself is of minimal material volume and weight and its design ensures safe and user-friendly handling during the operations that follow the weighing.